1. Field of the Invention
The present invention relates to a process for preparing an aqueous styrene-butadiene polymer dispersion by free-radical aqueous emulsion polymerization of a monomer mixture comprising styrene and butadiene by a monomer feed technique.
2. Description of the Background
Aqueous styrene-butadiene copolymer dispersions find diverse application, particularly as binders in coating compositions such as emulsion paints and colored paper coating slips, in barrier coatings, as a coating for the back of carpets, as an adhesive base material in carpet adhesives, in building adhesives, for modifying mortar, cement, and asphalt, for consolidating nonwovens, in sealants, in foam moldings, and as binders for leather dressing.
These dispersions are generally prepared by free-radical aqueous emulsion polymerization of monomer mixtures comprising styrene and butadiene. In the course of these processes, chain transfer agents are often used in order to prevent excessive crosslinking of the polymers, which can have an adverse effect on the performance properties of the dispersion. Such substances regulate the molecular weight of the polymer chains as they are forming, and are therefore also known as regulators.
The prior art proposes a very wide variety of different substances for use as regulators. Of commercial significance among these are compounds containing thiol groups, especially alkyl mercaptans such as n-dodecyl and tert-dodecyl mercaptan (see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. on CD-ROM, Synthetic Rubber 2.1.2). These substances are disadvantageous in a variety of respects, however; for example, because of their unpleasant odor, they are difficult to handle both before and during the polymerization. Another disadvantage is their effect on the inherent odor of the dispersions. This odor cannot be completely suppressed even by means of complex deodorization measures.
The prior art has variously proposed other regulators for the emulsion copolymerization of styrene with butadiene. In DE 195 12 999, for instance, regulators containing sulfur are used in combination with hydrocarbons such as α-methylstyrene dimer and terpinolene as regulators.
EP-A 407 059 discloses a process for the emulsion polymerization of monomer mixtures comprising styrene and butadiene, which uses mixtures of terpinolene in combination with other chain transfer agents.
It has now been found that terpinolene and other hydrocarbons which on abstraction of a hydrogen atom form a pentadienyl radical or a 1-phenylallyl radical as chain transfer agents, and also α-methylstyrene dimer alone, can be used as regulators. However, the dispersions obtained contain large amounts of organic compounds which are of low volatility and which in some cases cannot be polymerized. The amount of volatile hydrocarbons in the resulting dispersions, even following chemical deodorization of the dispersion, is generally above 3000 ppm and frequently above even 10 000 ppm. By chemical deodorization the skilled worker understands a postpolymerization process which is initiated by free radicals and carried out under forced polymerization conditions (see, for example, DE-A 44 35 423, DE-A 44 19 518, DE-A 44 35 422 and literature cited therein).
The volatile hydrocarbons are primarily a result of the hydrocarbon regulator and low molecular mass, unpolymerizable reaction products of the styrene and of the butadiene, such as ethylbenzene, 4-vinylcyclohexene, 4-phenylcyclohexene, and also unpolymerized monomers, especially styrene (i.e., residual monomer content) and unpolymerizable impurities in the feedstocks. High residual monomer contents are encountered especially when the amount of styrene in the monomer mixture to be polymerized is 40% by weight or more, and become all the more serious at styrene contents above 45% by weight, especially above 50% by weight, and in particular above 55% by weight. Although high levels of volatile constituents can sometimes be removed by subsequent physical deodorization, the expenditure, not least the expenditure in terms of time, and hence the costs, rise as the residual monomer content goes up. In particular the hydrocarbons that are used as regulators, however, can no longer be removed by conventional methods. Since, moreover, physical deodorization may have adverse consequences for the quality of the dispersion, a low level of volatile organic impurities prior to deodorization is desirable from the standpoint of quality as well.